1. Field of the Invention
The present invention relates to a novel method for in-situ conversion of hydrocarbon-bearing material and, more particularly, to such a method which allows for the controlled thermal linking of an injection well and a production well.
2. Setting of the Invention
In a practical sense, in-situ combustion methods to recover hydrocarbons from coal, tar sands or oil shale from underground formations have some control problems. Once a combustion zone has been initiated, the temperatures reached within the zone, the rate of travel and the exact direction of the zone may be difficult to control.
In a reverse combustion method to convert hydrocarbons, an oxygen, air or oxygen-containing gas or mixture thereof is introduced through an injection well and a combustion zone is established at a production well which moves toward the oxygen source at an injection well. A disadvantage of reverse combustion is that the heat losses to the formation may cause the reverse combustion zone progress to stall and then change into a forward mode, which may greatly reduce the amount of hydrocarbons recovered. The combustion zone will stop progressing against the flow of oxygen-containing gas and change to a forward mode and progress back towards the production well. Another disadvantage of reverse combustion is that a premature forward combustion mode can result from spontaneous ignition caused by the low temperature injected oxygen.
Reverse combustion suffers from another disadvantage, in that the procedure requires sufficient flux of the injected fluid. Flux can be defined as the volume of injected fluid per unit of time, per unit of area through which the fluid flows. There are two obstacles to the generation of this flux. First, the bitumen deposits are typically shallow so that the injection pressure and therefore the flux is limited. Exceeding this pressure limitation causes unnatural parting of the formation and subsequent loss of control. Secondly, the most desirable bitumen deposits, from an economic standpoint, are those containing the highest bitumen saturation. Unfortunately, the higher the bitumen saturation is, the lower the effective permeability to injected gas. Since the flux of the injected fluid is dependent on this gas permeability, it is therefore inversely proportional to the bitumen saturation. It can be seen that there is a need for a controlled process of in-situ combustion.